Update modernbertdecoder docs (#39453)
* update docs with paper and real model
* nit
* Apply suggestions from code review
Thanks to @stevhlui!
Co-authored-by: Steven Liu <59462357+stevhliu@users.noreply.github.com>
* Remove usage examples, add quantization
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Co-authored-by: oweller2 <oweller2@dsailogin.mgmt.ai.cluster>
Co-authored-by: Steven Liu <59462357+stevhliu@users.noreply.github.com>
